Field of the Invention
The present invention relates to an optical system with a diffractive optical element, and more particularly relates to an imaging optical system that is used in an optical apparatus such as a still camera and a video camera.
Description of the Related Art
As a method of reducing (correcting) a longitudinal chromatic aberration or a chromatic aberration of magnification that occurs in an imaging optical system used for a typical camera, a method of using a diffractive optical element is known. This is a method of using a negative dispersion characteristic (νd=−3.453) or strong abnormal dispersion (θgF=0.296) of the diffractive optical element different from those of a common glass material to obtain a strong achromatic effect that cannot be obtained by a common optical glass material.
Although it is possible to correct a chromatic aberration that occurs in the imaging optical system by using the diffractive optical element, a flare that occurs by unnecessary diffracted light of orders other than a designed order reaching an image plane may increase if a refractive power on a diffractive surface of the diffractive optical element is too strong in order to emphasize an aberration correction. In the imaging optical system using the diffractive optical element, in order to suppress the generation of the flare, Japanese Patent Laid-open No. 2007-121440 and Japanese Patent Laid-open No. 2011-002555 disclose arranged location and configurational conditions of diffractive optical element. In Japanese Patent Laid-open No. 2007-121440, a glass material with an abnormal dispersion within a range of a specific material property is located at an appropriate position in a first lens unit disposed at the most object side, and accordingly a satisfactory aberration correction effect is obtained. In addition to that, the diffractive surface is arranged at the imaging plane side so as to have a configuration where external light other than the imaging light does not easily enter the diffractive surface directly, and accordingly the generation of the flare caused by the diffractive surface is suppressed. In Japanese Patent Laid-open No. 2011-002555, a diffractive optical unit including a plurality of laminated diffractive gratings and a refractive optical unit formed by a solid material contact closely with each other to constitute a diffractive optical element, and the solid material of the refractive optical unit has an abnormal dispersion property within a specific material property. Furthermore, a thickness of the refractive optical unit in an optical axis direction is set within an appropriate range. As a result, the aberration correction effect is enhanced, and a refractive power on the diffractive surface of the diffractive grating unit is relaxed to suppress the generation of the flare.
However, in any of Japanese Patent Laid-open No. 2007-121440 and Japanese Patent Laid-open No. 2011-002555, the refractive power on the diffractive surface of the diffractive optical element is considered, but they only consider a refractive power specified by phase coefficients of low order terms in a phase function. According to the study by the inventor, when considering the flare caused by the diffractive surface of the diffractive optical element, the refractive power specified by the phase coefficients of the low order terms is insufficient and a refractive power specified by phase coefficients of high order terms need to be considered.